The present invention relates to a method and an apparatus for compressing a video signal.
A method of compressing a video signal is to divide the video signal into blocks and code them by an orthogonal transformation.
A sampling pattern for explaining a conventional method of compressing a video signal is shown in FIG. 1, in which pixels are arranged in matrix. Circles indicate positions of pixels, and dashed lines indicate block boundaries for coding by an orthogonal transformation. In this conventional method, 16 pixels are included in each block. Also, character H1 designates a horizontal conversion coordinate axis for the orthogonal transformation of a video signal, and character V1 a vertical transformation coordinate axis for the same purpose.
In the orthogonal transformation of a video signal divided into blocks as shown above, the coding by the transformation is effected along the coordinate axes H1 and V1 crossing at right angle to each other. Signals coded by the transformation represent two-dimensional frequency information of the coordinate axes H1 and V1. According to this two-dimensional frequency information, the redundant information such as high-frequency or diagonal components of the original video signal are reduced by use of the statistical or visual characteristics of the signal, thereby compressing the amount of information.
The orthogonal transformation includes Hadamard transformation or discrete cosine transformation (DCT).
FIG. 2 is a diagram showing a sampling pattern for explaining a second conventional method of compressing a video signal, in which pixels are arranged in quincunx or non-matrix having an interline offset. Circles indicate positions of pixels.
This pixel arrangement is used for MUSE signal proposed for use with satellite broadcasting of a high-definition television signal. In this pixel arrangement, the frequency band in the diagonal direction is reduced by half that in the horizontal or vertical direction, and is also used for the original video signal in matrix subsampled as a compression means by reduction of diagonal components of a video signal.
In FIG. 2, dashed lines represent block boundaries for coding by an orthogonal transformation. Each block contains 16 pixels in this embodiment. Character h2 designates a horizontal conversion coordinate axis for the orthogonal transformation of a video signal, and character V2 a vertical coordinate axis for the same purpose. The conversion coordinate axis V2 is not linear since the sampling pattern is non-matrix.
FIG. 3 is a sampling pattern diagram for explaining a third conventional method of compressing a video signal, in which pixels are arranged in quincunx having an interline offset like the second conventional method. In FIG. 3, circles indicate pixel positions, and X interpolated pixels obtained from the values of pixels indicated by circles. Dashed lines represent block boundaries for coding by the orthogonal transformation. In this conventional method, each block contains 16 pixels and 16 interpolated pixels. Character H3 designates a horizontal conversion coordinate axis for the orthogonal transformation of the video signal, and character V3 vertical conversion coordinate axis used for the same purpose.
As explained above, when a video signal divided into blocks is subjected to the orthogonal transformation, the coding by the transformation is effected along the conversion coordinate axes H3 and V3. The conversion coordinate axis V3, however, assumes a linear form since the sampling pattern is of not non-matrix but matrix due to the interpolated pixels.
In the second conventional method, the conversion coordinate axis V2 is not linear and therefore the conversion coordinate axes H2 and V2 fail to cross at right angles to each other. The signal coded by the transformation thus contains a frequency component of the conversion coordinate axis H2 along the vertical conversion coordinate axis V2. Specifically, the frequency information along the conversion coordinate axis H2 is added to that along the vertical conversion coordinate axis V2, thereby preventing an efficient compression of a video signal.
The third conventional method, on the other hand, which has a linear conversion coordinate axis V2 and the conversion coordinates H2 and V2 crossing at right angles to each other, uses interpolation pixels X and therefore requires the coding by transformation of a video signal of 32 pixels for a block including 16 interpolation pixels even in an area corresponding to a video signal of 16 pixels. In other words, the object pixels are increased in number, thus leading on an increased scale and a reduced efficiency of compression processong.